I have the same issue and tried several times to understand why and how this bug was occuring, but I found nothing really useful.
Everything I know so far is that this "cut" of the light seem to be occuring way more frequently on custom ADT than Blizzard's ones and if you increase the Z axis of the light, the cut seem to - more on Blizzard ADT than custom one - be occuring only on the edge of your line of sight.
Still don't know why and how, I bet it's something related in the shaders, You could take a look at the flare (the hunter's spell) and look how it react, I never tried this guy on dark nights.
btw : you added light on your lampost m2 right ?
Hope it helps.